Zeolite ZSM-5 is a kind of high silicon zeolite with Pentasil structure developed by Mobil Oil Corporation (U.S. Pat. No. 3,702,886) and is widely used as catalytic material in isomerization, disproportionation, catalytic cracking and catalytic dewaxing due to its unique pore structure. U.S. Pat. No. 4,107,224 discloses a process for preparing ethylbenzene by the alkylation of benzene with ethylene utilizing zeolite ZSM-5 catalyst, wherein the raw material ethylene needs to be diluted with inert hydrocarbons. For the sake of industrial application, diluted ethylene is obtained from the tail gases of some devices in an oil refinery, such as dry gas of catalytic cracking, coke oven gas and the like. However, when such tail gases are used as sources of diluted ethylene, they require to be pretreated to remove the impurities therein such as hydrogen sulfide, water, carbon dioxide and the like until the contents of them are below 10 ppm.
Zeolite having a structure intermediate that of ZSM-5 and ZSM-11(hereafter refered to as a ZSM-5/ZSM-11 intermediate), another zeolite product developed by Mobil Oil Corporation (U.S. Pat. No. 4,229,424), is prepared by using quaternary ammonium salt as template. The ZSM-5/ZSM-11 intermediate has a characteristic X-ray diffraction pattern and such structures intermediate that of ZSM-5(designated as I) and ZSM-11(designated as S) as SISI, SSII, IISIIISI, SSSISI, IISISI, etc., and it has, in anhydrous state, a chemical formula in terms of mole ratios of oxides as follows: 0.9.+-.0.3 M.sub.2/n O:Al.sub.2 O.sub.3 :ZSiO.sub.2, wherein M is a cation, preferably hydrogen or a hydrogen precursor, e.g, ammonium cation; n is the valence of the cation; and Z is at least 5 and may be up to 5000. It is taught in the patent that the cations in the zeolites can be replaced, at least in part, by ion exchange with other cations in accordance with techniques well-known in the art. U.S. Pat. No. 4,289,607 further discloses the use of said zeolite in catalytic reactions, such as the conversion of methanol to gasoline, oligomerization of olefins, alkylation of aromatics, isomerization of xylene, catalytic cracking of hydrocarbons and the like.
In general, incorporation of rare earth elements into zeolites will increase the activity and stability of the zeolites. For example, REY zeolites obtained by ion-exchange treatment of NaY zeolites with RE.sup.3+ cation have higher cracking activity and hydrothermal stability than those of NaY zeolites (U.S. Pat. No. 3,402,996). However, with respect to such high silicon zeolites as ZSM-5, ZSM-11 and ZSM-5/ZSM-11, due to their high silica to alumina ratio and therefore less electric charge centers and high hydrophobility as well as smaller pore openings in the channel of zeolite, it is very difficult to incorporate such trivalent cations of large ionic radius as rare earth cations into the channel of those zeolites in aqueous medium by conventional ion-exchange techniques, thus RE.sup.3+ is less likely to coordinate with the electronegative centers of aluminium on the low density skeleton (P. Cho and F. G. Dwyer, ACS Symp. Ser., 218, 59-78, 1983). GB2,033,358A discloses the preparation of zeolite ZSM-5 containing metal, including trivalent metal, by adding salts of mineral acids including salts of trivalent metals to the raw materials of zeolite synthesis. And CN 89108836.9, CN 90104732.5 and U.S. Pat. No. 5,232,675 discloses the synthesis of high silicon ZSM-5 zeolites containing rare earth elements by employing rare earth-containing faujusite as crystal seed. So far, these have been the reports known to incorporate rare earth into ZSM-5 zeolite by synthesis instead of conventional ion-exchange techniques. But it has never been reported how a rare earth-containing zeolite with a cocrystalline structure of ZSM-5 and ZSM-11 can be directly synthesized.
An object of the invention is to provide a directly synthesized rare earth-containing zeolite with a cocrystalline structure of ZSM-5 and ZSM-11. Another object of the invention is to provide a process for preparing the zeolite. A further object of the invention is to provide uses of the zeolite in catalytic conversions.